Re: [PATCH RFC] nvme-multipath: fix diskstats for partitions

[John Garry <john.garry@linux.dev>]

On 7/24/26 16:37, Keith Busch wrote:
> On Fri, Jul 24, 2026 at 04:29:08PM +0100, John Garry wrote:
>> On 7/24/26 13:45, Keith Busch wrote:
>>> Can we just thread through partitions for the bio's block_device instead
>>> of assuming part0? I know the hidden path devices skip partition
>>> scanning, but maybe if we let it happen, then those will have the same
>>> partition setup as the head gendisk. Then we can go right to the
>>> disk->part_tbl for what we provide to bio_set_dev() for both submission
>>> and failover, and everything should work out from there.
>>
>> I guess that we would just scan through the per-path gendisk->part_tbl and
>> match somehow to lookup the partition. Maybe vs start address of
>> bio->bi_bdev. Or is there a simpler (and quicker) way?
>>
> 
> This is the idea, assuming we can get the partition tables of the head
> and path disks to be aligned:
> 
> ---
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -543,7 +543,10 @@ static void nvme_ns_head_submit_bio(struct bio *bio)
>          srcu_idx = srcu_read_lock(&head->srcu);
>          ns = nvme_find_path(head);
>          if (likely(ns)) {
> -               bio_set_dev(bio, ns->disk->part0);
> +               struct block_device *bdev;
> +
> +               bdev = xa_load(&ns->disk->part_tbl, bdev_partno(bio->bi_bdev));
> +               bio_set_dev(bio, bdev);
>                  /*
>                   * Use BIO_REMAPPED to skip bio_check_eod() when this bio
>                   * enters submit_bio_noacct() for the per-path device. The EOD
> --

ok, got it.

And I think that we would need to do the reverse lookup in 
nvme_mpath_start_request() and nvme_mpath_end_request() to get the head 
disk partition, like:
bdev = xa_load(&ns->head->disk->part_tbl, bdev_partno(bio->bi_bdev));

I suppose that the tricky part now would be have the per-path disk scan 
run but keep those per-path disks hidden.

I experimented by stop setting GENHD_FL_HIDDEN for the per-path disk, 
and the diskstats look ok, FWIW.